combineLatest alternative to combine or merge Observables

Question

I was under the impression combineLast was a good fit at first, but as I read the docs it seems it isn’t: “Be aware that combineLatest will not emit an initial value until each observable emits at least one value.”… And of course I hit exactly that exception. I tried forkJoin and merge in various combinations, but I can’t get it right.

The use-case is pretty straightforward, the method someObs returns 0 or more observables, over which I loop. Based on a value on the SomeObs object I push a newly constructed observable, OtherObs[], to an Array of Observable<OtherObs[]>. This array "needs" to be merged into a single observable, and before returning it I’d like to do some transformations.

Specifically I’m having difficulty replacing the combineLast operator with something appropriate…

public obs(start: string, end: string): Observable<Array<OtherObs>> {
  return this.someObs().pipe(
    mergeMap((someObs: SomeObs[]) => {
      let othObs: Array<Observable<OtherObs[]>> = [];

      someObs.forEach((sobs: SomeObs) => {
        othObs.push(this.yetAnotherObs(sobs));
      });

      return combineLatest<Event[]>(othObs).pipe(
        map(arr => arr.reduce((acc, cur) => acc.concat(cur)))
      );
    })
  );
}

private yetAnotherObs(): Observable<OtherObs[]> {
  /// ...
}

private somObs(): Observable<SomeObs[]> {
  /// ...
}

Answer 1

The "problem" of combineLatest is (like you said) that it "will not emit an initial value until each observable emits at least one value". But this is not a problem because you can use the RxJS operator startWith.

So your observable gets an initial value and combineLatest works like a charme ;)

import { of, combineLatest } from 'rxjs';
import { delay, map, startWith } from 'rxjs/operators';

// Delay to show that this observable needs some time
const delayedObservable$ = of([10, 9, 8]).pipe(delay(5000));

// First Observable
const observable1$ = of([1, 2, 3]);

// Second observable that starts with empty array if no data
const observable2$ = delayedObservable$.pipe(startWith([]));

// Combine observable1$ and observable2$
combineLatest(observable1$, observable2$)
  .pipe(
    map(([one, two]) => {
      // Because we start with an empty array, we already get data from the beginning
      // After 5 seconds we also get data from observable2$
      console.log('observable1$', one);
      console.log('observable2$', two);

      // Concat only to show that we map boths arrays to one array
      return one.concat(two);
    })
  )
  .subscribe(data => {
    // After 0 seconds: [ 1, 2, 3 ]
    // After 5 seconds: [ 1, 2, 3, 10, 9, 8 ]
    console.log(data);
  });

See example on Stackblitz